As well as personal convictions that are so strong they are faily non-negotiable, most of us have preferences about what we'd like. Some people love going on long hikes, and really want someone to do that with. If so, someone like me with a serious back problem is not going to be ideal for them. I have some preferences related to children. I'd like to have at least four kids (biological or adopted), and I'd like to adopt at least one child from another country. However, these preferences are not on the same level as convictions. The events of life itself could dictate that preferences do not occur, even if both the husband and the wife are in agreement.
Looking back on this, I realise that some things I'd considered preferences are actually convictions. If I am really honest (rather than trying really hard not to be too picky) I would have struggled enormously with marrying someone who would "prefer" to deliberately have few children. Why? I am actually convicted that this type of preference does not sufficiently take into account the Bible's teaching about children and sexuality.
I don't have any easy answer for how to discern between convictions and preferences. However, a useful guide may be to honestly assess whether or not you believe the Bible speaks to a particular area. If you do, your view is probably a conviction rather than a preference. Hair colour, on the other hand, can be firmly placed in the preference category! Remember also that every person is a work in progress. The person you are considering courting may not be as convicted as you are in a particular area. However, you may grow together. Dave and I have both changed as we've responded one another's viewpoints and experiences.